mktemp: correct error message on absolute path

Correct the error message produced by `mktemp` when `--tmpdir` is
given and the template is an absolute path:

    $ mktemp --tmpdir=a /XXX
    mktemp: invalid template, '/XXX'; with --tmpdir, it may not be absolute

tests/by-util/test_mktemp.rs

@@ -414,6 +414,22 @@ fn test_mktemp_directory_tmpdir() {
     assert!(PathBuf::from(result.stdout_str().trim()).is_dir());
 }
 
+/// Test that an absolute path is disallowed when --tmpdir is provided.
+#[test]
+fn test_tmpdir_absolute_path() {
+    #[cfg(windows)]
+    let path = r"C:\XXX";
+    #[cfg(not(windows))]
+    let path = "/XXX";
+    new_ucmd!()
+        .args(&["--tmpdir=a", path])
+        .fails()
+        .stderr_only(format!(
+            "mktemp: invalid template, '{}'; with --tmpdir, it may not be absolute\n",
+            path
+        ));
+}
+
 /// Decide whether a string matches a given template.
 ///
 /// In the template, the character `'X'` is treated as a wildcard,